Was that the right folder to place my roms in? They are from a 0.78 set - emulation station can see them but not attract mode?
I can't help you with attract-mode setup, I'm not using it. I think you need to add the folder yourself and scan it for ROMs.
I did wonder if the system volume at 0% was the issue but that's something that just wont save when I set it higher?
If you disabled the on-board sound and your USB soundcard is the only output option, then the in EmulationStation the Audio Card should be set to 'Defaut' and the Default Device to PCM or Master (?). But the settings here affect only EmulationStation and not AttractMode or the emulators, so if you're not using EmulationStation, it's ok to leave them at defaults.